The removal of sulfur contaminants, specifically mercaptans, from hydrocarbon streams using caustic is known. Likewise, the oxidation of these mercaptans to disulfides by contacting the rich caustic stream with a catalyst in the presence of oxygen followed by separation of the disulfides from the treated caustic is also known. For economic reasons the treatment of spent caustic and subsequent recycle of the regenerated caustic is important. Likewise, reducing the need for excess equipment and the resultant saving of land space are continuing desirable goals.
Typically, liquid-liquid contactors are employed for the caustic treatment of hydrocarbons and in some cases fiber-film contactors as described in U.S. Pat. Nos. 3,758,404; 3,977,829 and 3,992,156, all of which are incorporated herein by reference. Such processes are typically followed by a caustic regenerator process involving an oxidation reactor followed by one or more separation vessels. A typical process flow scheme for treating a hydrocarbon involves a first caustic treatment using at least one liquid-liquid contactor to extract the sulfur contaminants, typically mercaptans, from the hydrocarbon feed, which generates a “spent” caustic solution that is rich in mercaptan or so called “rich caustic,” separating the treated hydrocarbons in the contactor, oxidizing the rich caustic to convert mercaptans to disulfides (typically referred to as disulfide oils (“DSO”)) which generates an “oxidized” caustic solution, and then using a gravity separator to separate the DSO from the oxidized caustic solution. In some instances, a granular coal bed is used in conjunction with the gravity settling device as a coalescer to further assist in the separation of the DSO from the oxidized caustic. Once the DSO is removed, the regenerated caustic can be further processed and then recycled, where it is mixed with fresh make-up caustic and used in the liquid-liquid contactors to treat the hydrocarbon feed. More typically, a further polishing processing is required in order to reduce the unconverted mercaptans and residual DSO to preferably below 5 weight ppm as sulfur. The presence of substantial mercaptans in regenerated caustic is undesirable because it can cause a loss of extraction efficiency and presents a potential for downstream formation of disulfides. The presence of substantial DSO in regenerated caustic leads to undesirable re-entry or back extraction of DSO into hydrocarbon during the hydrocarbon-caustic extraction process.
Solvent washing is a known technology and is often used as a polishing step to extract residual DSO from caustic. However, due to mass transfer and equilibrium limitations, these solvent washing unit operations usually require multiple stages with higher capital and operating costs. Besides, solvent washing is ineffective to remove mercaptans from caustic. Similarly, centrifugal process and membrane separation suffer from high costs and inability to achieve less than 5 weight ppm sulfur.
Adsorptive polishing is another technology that can be used. Adsorptive desulfurization has been applied to remove sulfur compounds from hydrocarbons such as gasoline and diesel. Examples are shown in U.S. Pat. Nos. 7,093,433; 7,148,389; 7,063,732; and 5,935,422. However, the adsorbents reported in these patents and in other literature are ineffective in caustic media.
Therefore, there remains a need to develop a technology that can economically removes both disulfides and mercaptans from caustic to achieve less than 15 weight ppm sulfur, preferably less than 7.5 ppm.
My process uses a single column or vessel to oxidize and remove both insoluble disulfides and mercaptans from rich caustic feeds. Further, my process is extremely economical compared to traditional methods for removing residual sulfur compounds from caustic solutions by minimizing both capital and operating costs. These and other advantages will become evident from the following more detailed description of the invention.